1. Field of the Invention
This invention relates to a pipe coupling, and more particularly to a coupling for sealedly connecting metal pipes for use in the oil industry.
2. Description of the Prior Art
Many high-pressure oil wells at great depths have been developed in recent years. The oil-well pipes used in this type of oil wells call for couplings that surpass the API standards for threads (round and buttress) in such properties as tensile strength, airtightness, corrosion and erosion resistance. Of these properties, airtightness is particularly important. Leakage of inflammable gas may lead to a fire and explosion in an oil well and leakage of such poisonous gas as hydrogen sulfide may result in injury and death.
The airtightness of thread joints have conventionally been attained by minimizing the clearance thereat by specifying close machining tolerance, plating such soft malleable metal as tin on the threaded portion, and applying grease mixed with fine metal dust and calcium-lithium-based soap to fill the clearance left. The torque and the number of turns in screwing the pipes together must be carefully controlled, as well. Even if these measures are taken, gas leakage is difficult to prevent in the stringent environment involving high temperatures and pressures in which many oil wells are drilled recently unless the clearance between the external and internal threads is thoroughly eliminated. Thus, airtight joints of such special types as provide a metal-to-metal contact seal on pins (pipes to be connected) and a casing (coupling) or a Teflon ring in an annular groove cut in the threaded portion, have been in use.
Several variations are known for the former type. One example disclosed in the Gazette of Japanese Patent Publication No. 18096 of 1970 comprises a casing that has a peripheral shoulder provided with an inversely conical opening at each end. On the inner surface of the casing are provided a threaded portion and an unthreaded portion that extends between one end of the threaded portion and said shoulder. A pin (i.e., pipe) to be connected has a corresponding conical end having threaded and unthreaded portions on the outside thereof. When put together, the external thread on the conical surface of the pin and the internal thread of the inversely conical surface of the casing establish a metal-to-metal contact.
Another known example of a similar type comprises a casing that has a peripheral shoulder at each end which is provided with an inversely conical inner surface having a threaded portion and an unthreaded tapered tip portion. The inner surface of pins to be connected is shaped in the opposite way. With this coupling, a first seal is established by the contact between the conical and inversely conical surfaces, as in the case of the preceding example. In addition, a second seal is built up by the metal-to-metal contact between the expanded portions provided on the inclined surfaces of the pin and casing (see the Official Gazette Publication of Japanese Provisional Patent Publication No. 17125 of 1973).
Still another example disclosed in the Official Gazette of Japanese Provisional Patent Publication No. 113885 of 1981 has two peripheral shoulders with an inversely conical surface at the end thereof and near the threaded portion thereon. The inner surface of the pins to be connected thereby is shaped substantially in the opposite manner.
The conventional couplings described above can be categorized as the type in which the tip of a pin contacts the peripheral shoulder of a casing. The contact surface is inclined at a certain angle (approximately 75 degrees) with respect to the center axis of the pipes connected.
As such, most of thrust obtained by exerting torque on the threaded portions is absorbed at the tip. Besides, the yield point, which may be assumed as 63 kg/mm.sup.2, is too small as compared with the modulus of elasticity which may be estimated as 21,000 kg/mm.sup.2. As such, the amount of axial interference that may occur when compressed to the limit of elasticity will be not more than a few microns at the most.
Even if a high surface pressure may be obtained, no significant amount of deformation induced by the machining error and tensile force in the axial direction will be absorbed.
All of the conical-surface sealing methods described above share a common fundamental problem that the interference between the conical sealing surfaces will produce a reaction force that forces the pin and casing to shift radially away from each other. This will cause the tip portion to deform in a unique manner. Then, when subjected to an axial force or pressure exerted from the inside or outside, the conical sealing surfaces of the pin and casing may develop a relative radial displacement.
Another variation disclosed in the Official Gazzette Publication of Japanese Provisional Patent Publication No. 186,690 of 1982 differs from those described above in that the taper of the conical sealing surfaces is as gentle as approximately 1/10 and that the tip of the pin substantially does not come in contact with the peripheral shoulder of the casing.
With this type of coupling, the amount of interference need not be greater than approximately 1/10 of the displacement induced by the machining error and tensile force in the axial direction. Owing to the wedge effect of the conical outer surface of an elastic member, high pressure can be obtained at the sealing surface with extremely low stress.
In the aforementioned pin-and-casing contact type, the contacting portion at the tip offsets the wedge effect to the gently tapered sealing portion.